ASM - Load address

[+Ripdubski]

I'm learning assembly so be kind if this is dumb.

 

Whats the best way to determine the load address when writing a program? One thing I've read said to subtract 1.5K from highest ram location but that seems silly.. Wouldnt that limit the program size to 1.5k?

 

Enlighten me.

[Rybags]

Load address is up to you, but you need to consider the environment, e.g. whether it has to work with a Dos present, whether it works with Basic or a cartridge present.

 

The "top of memory - 1.5K" is more like the upper limit, the real figure is closer to 1K. On a >= 48K machine the Display List will start at $9C20 which is precisely 32 bytes less than 1K.

 

For something that's bootable, e.g. cassette or ATR image the lower limit is generally $480 - the first byte after the cassette buffer. Once a program is loaded it can use the cassette buffer as workspace if required.

 

For something that's to be loadable from Dos - that's a tricky one since there's so many Doses out there and the memory use varies. To be safe, probably $3800. Although Dos 2.x should tolerate partially overwriting the DUP since it's dynamically loaded and can be refetched.

 

For something that coexists with Basic it gets tricky because there might be a Basic program present also.

 

But since Atari is generally a single-tasking system, it's usual that a loaded program will have exclusive use of the system or at most only show consideration for Dos.

[+Ripdubski]

Ok, i was reading $3600. I was more interested in if there was a programatic way to do it?

[Rybags]

6502 doesn't lend well to relocatable execution. An alternate is to do multiple versions of a program with different load addresses.

[MaPa]

I use $2000 as load address in my prods but I rarely return to DOS (only FLOP disk mag intros).

[flashjazzcat]

$2000 is a safe address to start applications.

[+Ripdubski]

Thanks all.

[+Ripdubski]

I used $3800. I got my first assembly program in 25+ years working! Woot! I did learn the hard way that RTS was required before .END I had a lot of strange things occurring when the program would end. Thanks again.